Figure F50. Close-up photograph of Sample 183-1138A-88R-1 (Piece 10, 130-141 cm) showing a bulbous cavity interpreted to be part of the base of a spiracle. The lower margin of basement Unit 20 is mostly planar with a 2- to 3-mm-thick zone of black clays and an ~1-cm-thick slightly oxidized zone with a finer groundmass. In the center of the recovered boundary is a 3 cm × 5 cm bulbous cavity. The zone of finer groundmass wraps up and around the cavity, but the black clays do not follow. Within the cavity, the lava has a vesicularity of 35%-40% with extremely angular and almost rectangular vesicles mostly 1-3 mm in dimension.